Development and application of molybdenum isotopes as a tool for tracking the evolving redox state of the Precambrian ocean

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Oxygen is vital to sustain many forms of life on Earth. Unlike the present-day, when life first evolved the atmosphere and oceans contained essentially no oxygen. Various lines of evidence suggest that the oxygen content of the atmosphere only began to rise about 2.3 billion years ago. Until recently it was thought that this also led to oxygenation of the ocean (as in the present day). A more recent hypothesis suggests that, instead, the increase in oxygen led to the weathering of sulphide minerals on land, which resulted in increased riverine delivery of sulphur to the oceans. The oceans then became rich in toxic hydrogen sulphide rather than oxygen (similar conditions are found in the modern day Black Sea). In fact, the ocean may only have become oxygenated following a second, much later rise in oxygen about 700 million years ago. All of this has profound consequences for the evolution of the biosphere. It is in the oceans where early life first evolved and flourished. The early biosphere was dominated by bacteria and the first photosynthesising bacteria probably evolved at least by 2.7 billion years ago, before the first major rise in oxygen. One of the puzzles of the early biosphere is why this early evolution of oxygen-producing photosynthesisers did not lead to the rapid oxygenation of the surface Earth thereafter. It is also clear that higher life forms, such as plants and animals (and ultimately humans) only began to evolve much later when the oceans eventually became oxic. Why was there a delay in the oxygenation of the surface Earth? Why did the biosphere only evolve slowly early in Earth's history? One prominent recent hypothesis attributes these puzzling features of the ancient Earth to ocean chemistry. One of the key requirements of photosynthesising bacteria is nutrients, which are essentially the elements contained in fertilisers- phosphorous, nitrogen, and trace metals such as molybdenum (Mo). Before the oxygenation of the atmosphere, the oceans were probably rich in dissolved iron (which is soluble in oxygen-poor water), leading to the widespread precipitation of chemical sediments very rich in iron (so-called Banded Iron Formations or BIFs). These may have taken vital nutrients like phosphorous and trace metals with them, leaving very low concentrations behind for bacteria to use. After the initial oxygenation of the atmosphere, and particularly if the oceans became sulphidic, trace metals may also have been in scarce supply as many of them are precipitated in the presence of hydrogen sulphide. This is important as it may have limited photosynthesis and hence oxygen production, helping to explain the apparent delayed oxidation of the Earth's surface, and hence the slow evolution of the biosphere. However, these ideas remain controversial. Detailed studies are required to assess whether the conditions described above did in fact exist. It is also important to determine how widespread these conditions were and how they affected nutrient availability. This project will examine nutrient availability as recorded by BIFs, the global extent of the transition to a sulphide-rich ocean following the first rise in atmospheric oxygen, and the chemical evolution of the oceans in the subsequent period of Earth's history leading up to the major explosion of animal and plant life. The tool we will use is the isotopes of molybdenum. The oceanic chemistry of Mo, and specifically the processes by which it is removed from solution into sediments, is highly dependent on the chemical state of the oceans. Further, these removal processes have variable preferences for the different isotopes of Mo, which makes the record of Mo isotope variations in the rocks interpretable in terms of both the oxygenation state of the ancient oceans, and the availability of Mo as a nutrient. This research should ultimately provide a better understanding of the links between ocean chemistry and the evolution of life on Earth.